1. Field of the Invention
In the operation of nuclear power reactors and the reprocessing of spent nuclear fuel, radioactive waste products are obtained which must be handled with great care and disposed of safely by storing them for a long period of time in a geologically secure location, thus preventing any dispersal of such radioactive material into the biosphere.
The radioactive waste products may be in the form of particles or larger pieces of material. The particulate material may consist of highly radioactive consumed ion exchange compounds and residual products from fuel reprocessing operations which have been converted into solid form. The larger pieces of material may be, for example, parts of fuel element cladding tubes.
2. Description of Prior Art
One method for incorporating waste material into a solid body, which is resistant to leaching by water, is disclosed in the specification of Larker U.S. Pat. No. 4,172,807 (granted on Oct. 30, 1979 and assigned to the assignee of this application). Further details of such a method are disclosed in U.S. Pat. No. 4,409,028 granted Oct. 11, 1983 (and also assigned to the assignee of this application).
The radioactive waste material from filters and the high level radioactive waste from a reprocessing plant, after being converted into solid material, is mixed with a compound of a material resistant to leaching by water and filled into containers. The contents of the containers are pressed at a high isostatic pressure and at such a temperature (HIP) that bonding between the container contents is achieved, thus obtaining a dense, solid body. This high temperature compaction may be preceded by isostatic pressing at room temperature (CIP). Larger pieces, such as parts of cladding tubes, can be pressed with or without intermixing of particulate or granular material, which increases the fill factor but increases the resistance of the pieces prepared for storage to leaching by water.
During cold isostatic pressing (CIP) and/or hot isostatic pressing (HIP) of containers which are filled with material so that a relatively low fill factor is obtained, the container will be badly deformed as its wall is crushed in to fill the voids therein. If the fill factor is lower than 50%, the densification during hot isostatic pressing (HIP) generally takes place in an unstable manner, and this results in an unpredictable deformation of the container. The wall of the container can then be so badly deformed that it is subjected to a very high localized stress. These stresses may result in the container wall cracking and in the pressure medium used for generating the isostatic pressure penetrating into the interior of the container. The CIP and/or the HIP compression will then fail to achieve its intended purpose. This can be serious if the material being processed is expensive and is ruined by a container wall failure but is a near disaster if the material within the container is poisonous or radioactive and is spread with the pressure medium to contaminate the equipment during the subsequent pressure reduction.
In an attempt to avoid this problem it has been known when compressing material which has a low fill factor, to use a container with a corrugated bellows-like wall to facilitate the collapse of the container and reduce the risk of splitting of the container during compression. However, using a corrugated wall does not avoid high stresses developing at the junctions between the lid and the bottom and the bellows-like wall of the container.